1. Field of the Invention
This invention relates to metal halide lamps and the method for making the same, and more particularly to metal halide lamps used as light sources for liquid crystal projectors or head lamps for automobiles by taking advantage of its high color rendering properties.
2. Discussion of the Related Art
FIG. 2 of the accompanying drawings illustrates a conventional metal halide lamp 90 comprising bulb 91 formed of quartz glass enclosing discharge chamber 91a in which a pair of rod-shaped electrodes 92 is disposed such that one end of one electrode is opposite to an end of the other electrode.
One end of the electrode 92 projects a predetermined distance into the discharge chamber 91a to form light-emitting portion 92a. The other end of the electrode 92 is disposed within bulb 91 by pinch sealing to form a pinch-sealed portion 92b. The electrode is also connected to one of the metal foil elements 93 which is connected to an electrical conductive lead wire 94 that extends outside of the envelope of the lamp.
In the conventional metal halide lamp 90, several problems are encountered. Firstly, flaking of the electrode is promoted by the gaseous impurities, which are not consumed in the process, found in discharge chamber 91a, and the heating of the electrode. The flakes diffuse throughout the discharge chamber 91a and adhere to the internal surface of discharge chamber 91a, which can cause a decrease in the efficiency of transmission through the bulb 91, thereby decreasing its luminance. Secondly, the gaseous impurities in discharge chamber 91a may cause a decrease in the efficiency of metal halide lamp 90 by increasing the discharge starting voltage. Lastly, the heat generated by the operation of the lamp causes thermal expansion of pinch-sealed portion 92b of electrode 92, thereby requiring some means to prevent the sealed part of bulb 91 from cracking.